The present invention relates to an internal combustion engine that changes combustion modes. More particularly, the present invention pertains to a controller for reducing the torque of such an engine.
To improve fuel economy and to increase engine power at the same time, engines that change combustion mode according to current running conditions have been introduced. Japanese Unexamined Patent Publication No. 5-288098 discloses such an engine.
In that publication, the engine includes a fuel injector to supply fuel to each combustion chamber. When receiving a relatively great load, the engine must increase its power to increase the speed to a relatively high speed range. In this state, the engine performs homogeneous combustion, that is, it distributes and burns homogeneously mixed air-fuel mixture. Specifically, homogeneous combustion is performed by homogeneously mixing fuel injected during an intake stroke of a piston and igniting the mixture in the combustion chamber.
When the load applied to the engine is relatively small and the engine speed is low, the engine does not need to increase its power. In this state, the engine performs stratified combustion to enrich the air-fuel mixture near the spark plug. As a result, the mixture is easier to ignite. At the same time, the overall air-fuel ratio is adjusted to be slightly greater than the theoretical, or stoichiometric, air-fuel ratio, which improves the fuel economy. During stratified combustion, fuel is injected toward a recess formed on the head of a piston in the combustion chamber during a compression stroke, which concentrates fuel in the vicinity of the plug. The mixture of the concentrated fuel and the air in the combustion chamber is ignited by the spark plug.
Switching the combustion modes of an engine between homogenous charge combustion and stratified combustion improves the fuel economy and provides sufficient engine power when required.
The output torque of a vehicle internal combustion engine sometimes needs to be reduced in accordance with the current running conditions. For example, the output torque must be reduced when an automatic transmission coupled to the engine shaft is shifted. Shifting an automatic transmission produces shift shock. The shift shock is reduced by decreasing the engine output torque when the transmission is shifted.
Several methods of decreasing the output torque of engines are known. These include decreasing the amount of intake air, delaying the ignition timing and decreasing the amount of injected fuel. These methods may be applied to the above described engine, which switches the combustion modes. However, a given method of reducing the torque is not necessarily suitable for each combustion mode.
Delaying the phase of the ignition timing during stratified combustion destabilizes fuel combustion, which may lead to misfires. During stratified combustion, ignition must be executed when relatively a rich air-fuel mixture is in the vicinity of the spark plug. However, if the ignition timing is delayed, ignition is executed when the richer mixture is not near the spark plug.
During homogeneous combustion, decreasing the amount of injected fuel by a significant amount will lead to misfires if the amount of intake air is constant. Thus, the output torque cannot be significantly reduced by decreasing the injected fuel amount. That is, the output torque can be reduced only by a small amount by decreasing the injected fuel amount. As a result, the output torque cannot be reduced to the required level.
The above drawbacks are present not only when shifting an automatic transmission but also when a traction control apparatus commands the engine to decrease the output torque to keep a wheel from spinning and also when the engine output torque is reduced to reduce the shock caused by abrupt depression of the accelerator.